International study shows affect of green light on sleep

11:35 May 13, 2010

The research, published in the May 12 issue of Science Translational Medicine, has shown that green light is effective in eliciting non-visual responses to light, such as resetting circadian rhythms, affecting melatonin production and alerting the brain.

Monash University researcher and co-author Associate Professor Shantha Rajaratnam said previous studies had shown that blue light played an important role in impacting the body’s natural internal body clock.

“We have known for some time that blue light has an impact on the body’s release of hormones such as melatonin, which is connected to sleepiness, by affecting specialised photoreceptors in the eye. This has led to treatments being developed based on the use of blue light to influence response. The new results will see a change in how practitioners and researchers view the influence of light, knowing that green light also has a role to play,” Associate Professor Rajaratnam said.

In the human eye there is a novel photoreceptor system used for vision, which detects light and is responsible for non-visual responses, such as resetting the internal circadian body clock, suppressing melatonin release and alerting the brain.

These photoreceptors are located in the ganglion cell layer, where there is a specialised subset of cells which are specifically responsive to blue light. Previous research indicates these cells are the primary way in which light is detected for non-visual responses. The study shows that cone photoreceptors, which are used for colour vision and are most sensitive to green light, also play a role in eliciting non-visual responses.

Researchers enrolled 52 healthy volunteers in a nine-day study in the Intensive Physiological Monitoring unit at Brigham and Women’s Hospital, a teaching affiliate of Harvard Medical School in the United States. The environment was free from all time cues, including windows, clocks, internet and television. Researchers shifted the participants’ schedules so that they slept during the day and were awake at night, during which time they were exposed to 6.5 hours of either green or blue light to simulate an overnight work shift. The light exposure was timed to reset the internal circadian body clock later than normal, equal to the adaption required to prevent jet lag following a westward flight across time zones.

Researchers measured the effect of the light exposure on melatonin levels and the shift in the timing of the circadian clock. They found that while blue light is usually the most effective way to stimulate the non-visual responses – especially under bright light conditions – stimulation with green light was also capable of eliciting the non-visual responses under certain circumstances. At the start of the light exposure or when exposed to dim light, green light was equally as effective as blue light at stimulating these non-visual effects, but then the effects dissipated more quickly over time.

“Our findings suggest that by dynamically manipulating the colour, duration and pattern of light, current available light therapies could be optimized and new therapies could be developed. These findings have the potential to play an important treatment role for a number of disorders including circadian rhythm sleep disorders, seasonal affective disorder and dementia,” said Steven Lockley, PhD, senior author and researcher at Brigham and Women’s Hospital, and Adjunct Associate Professor in the School of Psychology and Psychiatry at Monash University.

This study was funded by the National Institutes of Health and the National Space Biomedical Research Institute.

Shantha Rajaratnam is Associate Professor in the School of Psychology and Psychiatry at Monash University. He is also a Lecturer in Medicine in the Division of Sleep Medicine at Harvard Medical School (USA) and Associate Neuroscientist in the Division of Sleep Medicine at Brigham and Women’s Hospital (USA). For more information about BWH visit www.brighamandwomens.org.

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